1. Field of the Invention
The present invention relates to a phase shifting mask and a manufacturing method thereof as well as an exposure method using such a phase shifting mask.
2. Description of the Background Art
In semiconductor integrated circuitry, so many developments have been made for high integration and miniaturization. Accordingly, the rapid development has been made for miniaturization of a circuit pattern which is formed on a semiconductor substrate (hereinafter referred merely to as a wafer).
Among other things, photolithography has been recognized widely as the basic technology for pattern formation, in which various developments and improvements have already been made. However, as miniaturization of patterns proceeds, an improvement of resolution of the pattern is strongly required.
Photography is a technology for transferring a mask (original) pattern on a photoresist applied on a wafer and patterning an underlying film to be etched. When transferring the photoresist, the photoresist is developed. During development, the photoresist of the type in which a portion exposed to light is removed is called a positive type photoresist, while the type in which a portion not exposed to light is removed is called a negative type. Now, a conventional exposure method utilizing the photolithography technology will be described.
FIG. 45 is a schematic diagram of an optical system for illustrating a conventional exposure method. In this optical system, referring to FIG. 45, a pattern on a mask is reduced and projected onto a photoresist placed on the wafer. The optical system includes an illumination optical system covering from a light source to a photomask pattern, and a projection optical system covering from the photomask pattern to the wafer.
The illumination optical system includes a mercury lamp 111 serving as a light source, a reflection mirror 112, a light collecting lens 118, a fly eye lenses 113, a diaphragm 114b, light collecting lenses 116a-116c, a blind diaphragm 115, and a reflection mirror 117. The projection optical system includes telephoto lenses 119a-119b and a diaphragm 125.
In exposure operation, a beam of light 111a emitted from mercury lamp 111 is reflected from reflection mirror 112, so that only a g-line (wavelength:436 nm), for example, is reflected to become a beam of light having a single wavelength. Beam of light 111a then enters each lens 113a constituting fly eye lens 113, and after that, passes through diaphragm 114b.
Light passes through a light path 111b produced by one lens 113a constituting the fly eye lens, and light passes through a light path 111c produced by fly eye lens 113.
Beam of light 111a transmitted through diaphragm 114b then passes through light collecting lens 116a, blind diaphragm 115 and light collecting lens 116b, and is reflected from reflection mirror 117 at a predetermined angle.
Reflected from reflection mirror 117, beam of light 111a transmits through light collecting lens 116c and irradiates uniformly the entire surface of a photomask 720 in which a predetermined pattern is formed. Then, beam of light 111a is reduced by projection lenses 119a, 119b by a predetermined times for exposing a photoresist 121a placed on a semiconductor wafer 121.
Generally, a resolution limit R (nm) in photolithography using the demagnification exposure method is represented as EQU R=k.sub.1 .multidot..lambda./(NA)
where .lambda. is a wavelength (nm) of the light used, NA is a numerical aperture of a lens, and k.sub.1 is a constant depending on a resist process.
As can be seen from the above expression, a method conceivable to improve resolution limit R, i.e., to obtain a fine pattern, is to reduce k.sub.1 and .lambda. values and increase an NA value. In other words, it is sufficient to reduce the constant depending on the resist process while shortening the wavelength and increasing NA.
However, improvement of light source or the lens is technically difficult, and a depth of focus .delta. of the lens (.delta.=k.sub.2 .multidot..lambda./(NA).sup.2) might be shallower by proceeding shortening of the wavelength and increasing of NA, thus causing deterioration of the resolution.
With this in view, studies for attempting miniaturization of patterns by improving the photomask are under way. Lately, a phase shifting mask has been focused as a photomask capable of improving a resolution of the pattern. Now, a structure and the principle of such a phase shifting mask will be described in comparison with an ordinary photomask. The below description will be directed to a phase shifting mask of a Levenson system and a halftone system.
FIGS. 46A-46C represent, respectively, a cross section of a mask when a conventional photomask is used, an electric field on the mask, and a graph showing the light intensity on a wafer. Referring to FIG. 46A, a metal mask pattern 503 is formed on a glass substrate 501 in the conventional photomask. The electric field on such a conventional photomask is pulse-modulated spatially by metal mask pattern 503 as shown in FIG. 46B.
However, as can be seen from FIG. 46C, as the pattern is miniaturized, exposure light transmitted through the photomask also enters an unexposed region (a region where the exposure light is intercepted by metal mask pattern 503) due to diffraction of light. Therefore, light is directed also to the unexposed region on the wafer, thus decreasing contrast of light (a difference in light intensity between the exposed region and the unexposed region on the wafer). Consequently, the resolution is decreased, making transfer of fine patterns difficult.
FIGS. 47A-47C represent, respectively, a cross section of a mask when a phase shifting mask of the Levenson system is used, an electric field on the mask, and a graph showing light intensity on a wafer. Referring to FIG. 47A, the phase shifting mask includes an optical member called a phase shifter 505 provided on the conventional photomask.
More particularly, a chromium mask pattern 503 is formed on glass substrate 501 so that exposure regions and shaded regions are provided, and phase shifters 505 are provided every other exposure region. Phase shifter 505 serves to convert a phase of the transmitted light by 180.degree..
Referring to FIG. 47B, since phase shifters 505 are provided every other exposure region as described above, the electric field on the mask is structured such that phases of the light transmitted through the phase shifting mask are inverted alternately by 180.degree.. Phases of light are thus reversed in adjacent exposure regions, so that beams of light are canceled with each other due to interference of light in a portion where reverse-phased beams of light are overlapped.
As a result, as shown in FIG. 47C, the light intensity is reduced at a boundary of adjacent exposure regions, so that a sufficient difference in light intensity between exposure regions and unexposure regions on the wafer can be secured. This allows improvement of the resolution for transferring fine patterns.
FIGS. 48A-48C represent, respectively, a cross section of a mask when a phase shifting mask of the halftone system is used, the electric field on the mask, and a graph showing light intensity on the wafer. Referring to FIG. 48A, an optical member called a phase shifter 506 is also provided in the phase shifting mask of the halftone system as in the above-described Levenson system.
Only difference is that optical member 506 is formed only on an opaque film 503 on glass substrate 501, so that a two-layered structure of phase shifter 506 and opaque film 503 is provided. Phase shifter 506 serves to convert the phase of the transmitted light by 180.degree. as described above, and opaque film 503 serves to decay the intensity of the exposure light without completely intercepting the exposure light.
Referring to FIG. 48B, since the two-layered structure of phase shifter 506 and opaque film 503 is provided as described above, phases of light are converted by 180.degree. alternately in the electric field on the mask, while at the same time the intensity of one phase becomes smaller than the other. More particularly, phases of light are converted by 180.degree. due to transmission through phase shifter 506, and the intensity of light is decayed due to transmission through opaque film 503 such that a predetermined thickness of the photoresist can remain after development. The phases of the light are reversed in adjacent exposure regions so that beams of light are canceled with each other in the region where reverse-phased beams of light are overlapped.
As can be seen from FIG. 48C, the intensity of light can be reduced at an edge of the exposure pattern because the phase is reversed at the edge. Consequently, the difference in the light intensity between the region where the exposure light is transmitted through opaque film 503 and the region the light is not transmitted through the film becomes greater, whereby the resolution of the pattern image can be improved.
As described above, there are many types of phase shifting masks including the Levenson system, the halftone system and the like. Among others, a good resolution can be obtained according to the principles of a so-called phase shifting mask of the Levenson system which had been invented by Marc Levenson, and that system is considered as the most favorable system from the standpoint of resolution.
Various techniques have been invented and proposed for manufacturing such a phase shifting mask, however, none has been used in practice. Among these proposals, one prior art technique which is considered superior is a manufacturing technique by Marc Levenson which is described in Marc D. Levenson et al., "Phase-Shifting Mask Strategies: Isolated Dark Lines" MICROLITHOGRAPHY WORLD, pp.6-12, March/April 1192.
Therefore, a structure and a manufacturing method of a phase shifting mask of the Levenson system according to the above technique will be described below as a conventional first phase shifting mask.
FIG. 49 is a cross sectional view schematically showing a structure of a conventional first phase shifting mask. Referring to FIG. 49, a conventional first phase shifting mask 720 includes a quartz substrate 701 and a light shielding film 703.
Trenches are formed with a predetermined depth on a main surface of quartz substrate 701. A region where the trench is not formed serves as a first light transmitting portion 701a, while a region where the trench is formed serves as a second light transmitting portion 701b. Light shielding film 703 is formed on quartz substrate 701 so as to cover a sidewall portion of the trench and to expose predetermined regions of first and second light transmitting portions 701a and 701b. Light shielding film 703 has transmittance of not more than 1%, and a thickness of about 1000 .ANG. when chromium (Cr) is used as a material.
First and second light transmitting portions 701a and 701b are structured such that phases of the exposure light transmitted through respective portions are converted by 180.degree.. Since the phases of exposure light transmitted through adjacent light transmitting portions are thus converted by 180.degree., the resolution can be improved, as described above.
A bottom wall of the trench is substantially perpendicular to a sidewall thereof.
Now, a manufacturing method of the first phase shifting mask will be described below.
FIGS. 50-57 are schematic cross sectional views showing in this order a manufacturing method of the conventional first phase shifting mask. First, referring to FIG. 50, a chromium film 705a is formed on a surface 701a of quartz substrate 701. A resist film 707a is applied on chromium film 705a. Resist 707a is then exposed with light and developed.
Referring to FIG. 51, a resist pattern 707 having a desired shape is formed through the above exposure and development. Using resist pattern 707 as a mask, anisotropic etching is carried out. Resist pattern 707 is then removed.
Referring to FIG. 52, a chromium film pattern 705 is thus formed wherein a shifter pattern is transferred.
Referring to FIG. 53, using chromium film pattern 705 as a mask, anisotropic etching is carried out on quartz substrate 701, whereby a trench is formed on a surface 701a of quartz substrate 701 for transferring the shifter pattern. Chromium film pattern 705 is then removed.
Referring to FIG. 54, first and second light transmitting portions 701a and 701b are thus formed in quartz substrate 701.
Referring to FIG. 55, a chromium film 703a is formed on the entire surface wherein first and second light transmitting portions 701a and 701b are formed. A resist film 709a is applied on chromium film 703a. Resist film 709a is then exposed with light and developed.
Referring to FIG. 56, a resist pattern 709 having a desired shape is formed through the above exposure and development. Using resist pattern 709 as a mask, anisotropic etching is conducted to form a light shielding film 703 which exposes desired regions of first and second light transmitting portions 701a and 701b. After that, resist pattern 709 is removed, thereby completing the conventional phase shifting mask 720 shown in FIG. 57.
In the above-described manufacturing method of the conventional phase shifting mask, resist films 707a and 709a are not applied directly on quartz substrate 701. Accordingly, compare to a manufacturing method of the phase shifting mask in which the resist film is directly applied on the substrate (Japanese Patent Laying-Open Nos. 4-355758 and 2-211450), this phase shifting mask includes an advantage that a defect such as is described below will easily be repaired.
As one example of the method in which the resist film is directly applied on the substrate, a manufacturing method of the phase shifting mask described in Japanese Patent Laying-Open No. 4-355758 will be described.
FIGS. 58-61 are schematic cross sectional views showing in this order the manufacturing method of the phase shifting mask described in the above document. First, referring to FIG. 58, after a phase shifting film 803 is formed on the surface of a quartz layer 801, a light shielding film 805 is formed on the surface of the film 803.
Referring to FIG. 59, after a resist pattern 807 is formed on light shielding film 805, light shielding film 805 is patterned by etching with using resist pattern 807 as a mask. Resist pattern 807 is then removed.
Referring to FIG. 60, a photoresist 809 is directly applied on the surface of the patterned light shielding film 805 and phase shifting film 803. After patterning photoresist 809 into a desired shape, phase shifting film 803 is etched with using resist pattern 809 as a mask.
Referring to FIG. 61, a trench 811 is formed in phase shifting film 803. After that, resist pattern 809 is removed, thereby completing the phase shifting mask.
In the manufacturing method of the phase shifting mask disclosed in the above document, a shifter pattern is formed in phase shifting film 803 after light shielding film 805 is patterned, whereby resist film 809 is directly applied on the surface of phase shifting film (substrate) 803 in the step shown in FIG. 60.
In a typical method for applying the resist film, a pinhole 809a which penetrates through resist film 809 is generated as shown in FIG. 62. If etching is carried out by using resist pattern 809 as a mask with such pinhole 809a generated, a configuration shown in FIG. 63 will be obtained.
Referring to FIG. 63, an etchant enters pinhole 809a, so that phase shifting film 803 located at the bottom of pinhole 809a is also removed by etching, thus generating a defect. If the phase shifting mask including such a defect is used for exposure of a wafer, the phase of exposure light is converted in the defect portion in addition to desired regions. Therefore, the resist film applied on the wafer cannot be exposed into a desired shape.
Therefore, when the resist film is applied directly on the substrate, the defect of the shifter is introduced directly into the substrate upon carrying out etching or the like with using the resist pattern as a mask.
On the contrary, in the manufacturing method proposed by Marc Levenson, as described above, the resist film is not formed directly on the quartz substrate. Thus, even if etching is carried out to the underlying layer by using the resist film wherein pinholes are generated as a mask, direct formation of the defect of the shifter in the substrate can be prevented.
More specifically, referring to FIG. 64, even if a pinhole 707a is generated in a resist pattern 707, since a chromium film 705 is lying under resist pattern 707, only chromium film 705 at the bottom of pinhole 707a is removed by etching after etching is carried out to the underlying layer by using resist pattern 707 as a mask.
As can be seen in the figure, a pinhole defect (clear defect) 705a generated at the bottom of pinhole 707a is not in quartz substrate 701 but in chromium film 705 lying thereon. Thus, pinhole defect 705a can be easily repaired by filling up by deposition of a carbon-based film 705c in accordance with an FIB (Focussed Ion Beam) method, as shown in FIG. 65.
In some cases, as can be seen from FIG. 64, a remaining defect (opaque defect) 705b is generated at a portion where the chromium film should have been removed by etching. However, since such remaining defect 705b is not the type of defect which is formed directly in quartz substrate 701, the defect can be repaired easily by removing by blow (melt) of irradiation of laser using a YAG laser, as shown in FIG. 65.
Thus, the conventional manufacturing method proposed by Marc Levenson has the advantage that defects can easily be repaired because the defect of the shifter is not formed directly in the substrate.
Now, a structure of a phase shifting mask of the halftone system will be described below as a conventional second phase shifting mask.
FIG. 66 is a cross sectional view schematically showing a structure of a conventional second phase shifting mask. As can be seen from FIG. 66, a conventional second phase shifting mask 920 includes a quartz substrate 901 and a semi-light shielding film 903. A trench is formed with a predetermined depth in a main surface of quartz substrate 901.
A region where the trench is formed serves as a first light transmitting portion 901a, while a region where the trench is not formed serves as a second light transmitting portion 901b. Semi-light shielding film 903 is formed on quartz substrate 901 so as to cover a sidewall portion of the trench and to expose a predetermined region of first light transmitting portion 901a. Semi-light shielding film 903 serves to reduce the intensity of exposure light transmitting through semi-light shielding film 903 to such an extent that a photoresist on the wafer is not photosensitized by the exposure light or a predetermined thickness of the photoresist is left after development.
First and second light transmitting portions 901a and 901b are structured such that phases of the exposure light transmitted through respective portions are out of phase by 180.degree.. As the phases of the exposure light transmitted through adjacent light transmitting portions are thus converted by 180.degree., the resolution can be improved as described above.
In the meantime, a bottom wall of the trench is substantially perpendicular to a sidewall thereof.
With using conventional first and second phase shifting masks, as described above, a higher resolution can be obtained compared to an ordinary photomask. However, there are some problems in the conventional first and second phase shifting masks such as difficulty in obtaining a desired pattern shape because of I! increase in complexity of a circuit pattern and II! generation of a defect during formation of a shift mask. We will discuss these problems more in detail in the following.
I! Increase in Complexity of the Circuit Pattern
The recent circuit patterns of semiconductor integrated circuits have been made smaller and become complex in order to obtain the semiconductor integrated circuits which has a large capacity and multiple functions. For instance, in a DRAM (Dynamic Random Access Memory), periodic patterns are provided densely (hereinafter referred merely to as a dense pattern) in each memory cell region, while circuit patterns having individual functions are isolated from each other (hereinafter referred merely to as an isolated pattern) in its peripheral circuit region.
If a circuit pattern in which a dense pattern and an isolated pattern are mingled is to be formed by the phase shifting mask of the Levenson system, a good resolution can be obtained in the dense pattern, while at the same time, the resolution is not so good in the isolated pattern. In the meantime, if the phase shifting mask of the halftone system is used, then the resolution is better in the isolated pattern than in the dense pattern.
FIGS. 67A and 67B illustrate the reason why the phase shifting mask of the Levenson system cannot be used to obtain the isolated pattern at a good resolution, wherein FIG. 67A shows a cross sectional view of the phase shifting mask of the Levenson system, and FIG. 67B shows the light intensity on the wafer when exposure is conducted by using the phase shifting mask shown in FIG. 67A.
Referring to FIGS. 67A and 67B, the isolated pattern exists away from the other circuit pattern. Thus, an exposure region 701b (hereinafter referred to as an isolated exposure region) constituting the isolated pattern is spaced apart by a considerable amount from an exposure region constituting the other circuit pattern. Accordingly, the exposure light directed onto the wafer after being transmitted through isolated exposure region 701b will not overlap the exposure light having a reverse phase which is transmitted through the other exposure region. This prevents the phase shifting mask effect of obtaining a good resolution by canceling reverse-phased beams of light with each other due to diffraction of light.
On the contrary, in the dense pattern, reverse-phased beams of light are overlapped in adjacent exposure regions, as shown in FIG. 47. This contributes to improvement of the resolution due to canceling of beams of light in the region where beams of light are overlapped.
Next, FIGS. 68A and 68B illustrate the reason why the phase shifting mask of the halftone system cannot be used to achieve a good resolution in the dense pattern, wherein FIG. 68A shows a cross sectional view of the phase shifting mask of the halftone system, and FIG. 68B shows the light intensity on the wafer when exposure is conducted by using the phase shifting mask shown in FIG. 68A.
Referring to FIGS. 68A and 68B, since circuit patterns are dense in the dense pattern, a plurality of exposure regions 901a, 901a are disposed in proximity to each other. A portion P.sub.3 is thus generated wherein beams of light of the same phase are overlapped with each other in adjacent exposure regions 901a. When the beams of light of the same phase are overlapped, the light intensity cannot be diminished at the edge (P.sub.3 portion) of the exposure pattern because of a small intensity of the reverse-phased light transmitted through exposure region 901b. In this case, the resolution cannot be improved due to an insufficient difference of light intensity between the exposure region and the unexposure region.
On the contrary, in the isolated pattern, a portion where beams of light of the same phase are overlapped is not generated, as shown in FIG. 48. This contributes to an improvement of resolution because a sufficient difference in light intensity can be obtained at the edge of the exposure pattern due to overlap of the reverse-phased beams of light.
Therefore, in the pattern wherein the dense pattern and the isolated pattern are mingled, either one pattern cannot be formed with a good resolution even though the conventional first or second phase shifting mask is used. This prevents formation of a desired pattern in such a complex circuit pattern as having the dense pattern and the isolated pattern.
II! Generation of a Defect During Formation of a Shifting Mask
In the conventional manufacturing method proposed by Marc Levenson in which a shifter pattern is formed in quartz substrate 701, anisotropic etching is carried out to quartz substrate 701. In this respect, there is a problem of difficulty in obtaining a predetermined shape pattern in this manufacturing method because of the reasons such as (1) adherence of the light shielding film to the quartz substrate is degraded, (2) foreign objects are likely to remain on the quartz substrate, and (3) disadvantage due to remaining defects are easily generated.
(1) Adherence of the Light Shielding Film to Quartz Substrate
In the conventional manufacturing method of the phase shifting mask, anisotropic etching is conducted to quartz substrate 701 in the steps shown in FIGS. 52 and 53. The sidewall of the trench which is formed by the etching is thus substantially perpendicular to the bottom wall of the trench. In the step shown in FIG. 55, chromium film 703a is formed also to cover the trench. However, it is difficult to form the film appropriately on the sidewall portion of the trench having the above-described structure. Specially, if chromium 703a is formed by a method having a poor step coverage such as sputtering, the appropriate formation of the film becomes more difficult.
This leads to deterioration of adherence of chromium film 703a against quartz substrate 701 on the sidewall portion of the trench (a region indicated by D.sub.3 in the figure). When the adherence of chromium film 703a is not favorable, chromium film 703a will be peeled off easily during the cleaning step of the manufacturing process of the phase shifting mask. Also, light shielding film 703 will be peeled off easily during cleaning after completion of the phase shifting mask.
A portion where light shielding film 703 is thus peeled off becomes a so-called clear defect. When exposure is carried out onto the wafer by using the phase shifting mask having the clear defect, a region which should not be exposed will be exposed with light on the wafer, thus preventing formation of a pattern having a desired shape.
(2) Remaining of Foreign Objects on the Quartz Substrate
As described above, according to the manufacturing method of the conventional phase shifting mask, quartz substrate 701 is subjected to anisotropic etching in the step shown in FIGS. 52 and 53, so that the bottom wall of the trench formed during the etching is substantially perpendicular to the sidewall of the trench, as shown in FIG. 70. Since an edge of the stepped portion (a region indicated by D.sub.4 in the figure) is substantially 90.degree., a foreign object 750a is easily be trapped at the edge.
More specifically, after formation of the trench on the surface of quartz substrate 701 in the steps shown in FIGS. 53 and 54, a chromium film pattern 705 is removed. During this removal, a foreign object is trapped at the edge of the stepped portion. Also, foreign objects such as being generated internally from the etching apparatus or included in an etching solution can be trapped.
If a light shielding film is formed with foreign objects being trapped, an etchant spreads under a resist 709 through foreign object (or after melting the foreign object at a high speed) during patterning of light shielding film 703, as shown in FIGS. 71 and 72. Thus, light shielding film 703 is removed excessively by etching as shown in FIG. 73. This excessively-removed portion by etching becomes a so-called clear defect. A region which should not be exposed will be exposed with light on the wafer due to the clear defect, thus preventing formation of a pattern having a desired shape.
As can be seen from FIG. 73, a portion of light shielding film 703 indicated by an arrow K is not in contact with the sidewall of the stepped portion but is protruding into the space. This portion K of light shielding film 703 is easily be peeled off by cleaning after removal of the resist or by cleaning after repairing of the defect of light shielding film 703, thus generating the clear defect as in the above.
In the meantime, as shown in FIG. 74, the surface of light shielding film 703 which is formed on the edge of the stepped portion reflects the stepped shape of the underlying layer. Thus, a foreign object 750b will easily be trapped at a portion along the edge of the stepped portion of light shielding film 703 (a region indicated by D.sub.5 in the figure), as in the above.
If the trapped foreign object 750b is large, foreign object 750b will remain penetrating onto the light shielding portion in some cases. If foreign object 750b is made of a material through which the exposure light cannot be transmitted, the foreign object will necessarily be a so-called opaque defect. Even if the exposure light can transmits through foreign object 750b, a proper function of light shielding mask 703 as a phase shifting mask will be prevented when a phase of the transmitted light is shifted by a considerable amount (usually 10.degree.-20.degree.) or more by the material. Thus, the shape to be transferred onto the resist on the wafer is deformed, thus generating a defect.
Thus, when the foreign object is trapped in the stepped portion or the like in quartz substrate 701 during processing of the phase shifting mask, it is difficult to expose the resist on the wafer into a desired shape.
(3) Disadvantage Caused By Remaining Defects
In some cases, during patterning of chromium film 705a shown in FIGS. 50 and 51, a remaining defect 705b is generated as shown in FIG. 75. Such a remaining defect 705b can be repaired by the above-described laser blow or the like. However, this does not mean that all of generated remaining defects 705b cannot be sensed and repaired. Also, it is sometimes desired to omit a step of repairing for the sake of simplifying of a manufacturing process. In such a case, some remaining defects 705b are still left.
In the manufacturing method of the conventional phase shifting mask, the trench is formed on the surface of quartz substrate 701 by anisotropic etching in the steps shown in FIGS. 52 and 53. However, if anisotropic etching is carried out with remaining defect 705b being generated, an unetched region (a region indicated by D.sub.6 in the figure) which should have been etched is generated as shown in FIG. 76.
Referring to FIG. 77, the phases of the beams of exposure light transmitted through adjacent light transmitting portions are not converted by 180.degree. in the thus formed phase shifting mask. More particularly, the exposure light transmitted through light transmitting portion 701a and the exposure light transmitted through a light transmitting portion 701ab have the same phase. Accordingly, the exposure light transmitted through both transmitting portions 701a and 701ab will be intensified in a portion where those beams of light are overlapped. Consequently, a difference of light intensity between the exposed region and the light-shielded region on the wafer becomes small, so that the resolution is degraded, and formation of a desired pattern cannot be achieved.
Additionally, as shown in FIG. 78, if region 701ab which is not etched due to the remaining defect is generated partially at the light transmitting portion, the shape of transfer pattern will be deformed.
More particularly, phases of exposure light transmitted through unetched region 70lab and a region 701bb which is removed by etching are reverse. Accordingly, at an interface P between unetched region 701ab and etched region 701bb, a portion having the light intensity of zero is generated due to canceling of exposure light.
The resist on the wafer cannot be exposed with light at such portion having the light intensity of zero. In other words, a region where exposure should have been carried out is generated on the resist, so that a resist pattern having a desired shape cannot be obtained. If patterning of the underlying layer is carried out by using such a resist pattern, an insufficient pattern is formed.
Specially, when a negative type resist in which a portion shielded from light is removed by a developing solution is used, resist will not be left at a portion having the light intensity of zero (region P). That is, a region wherein resist should have been left is generated. When an interconnection layer, for example, is formed by patterning with using such a resist pattern, the interconnection will be cut off at region P having the light intensity of zero, as shown in FIG. 79.
Thus, in addition to deterioration of the resolution, the pattern shape will be deformed when the remaining defects are generated.